Chapter 2 Student Reading - Changing State
Changing from a solid to a liquid—Melting
In solids, the atoms or molecules that make up the substance have strong attractions to each
other and stay in fixed positions. These properties give solids their definite shape and volume.
When a solid is heated, the motion of the particles (atoms or molecules) increases. The atoms or
molecules are still attracted to each other but their extra movement begins to compete with their
attractions. If enough energy is added, the motion of the particles begins to overcome the
attraction and the particles move more freely. They begin to slide past each other as the
substance begins to change state from a solid to a liquid. This process is called melting.
The particles of the liquid are only slightly further apart than in the solid. (Water is the
exception because the molecules in liquid water are actually closer together than they are in ice)
The particles of the liquid have more kinetic energy than they did as a solid but their attractions
are still able to hold them together enough so that they retain their liquid state and do not
become a gas.
Different solids melt at different temperatures
The temperature at which a substance begins to melt is called the melting point. It
makes sense that different substances have different melting points. Since the
atoms or molecules that substances are made of have a different amount of
attraction for each other, a different amount of energy is required to make them
change from a solid to a liquid. A good example is the melting point of salt and
sugar. The melting point of sugar is 186 °C. The melting point for regular table
salt is 801 °C. Metals like iron and lead also have different melting points. Lead
melts at 327 °C and iron melts at 1,538 °C.
Some solids, like glass do not have a precise melting point but begin to melt over a range of
temperatures. This is because the molecules that make up glass are not arranged in as orderly a
way as those in crystals like salt or sugar or metals like iron. Depending on the type of glass, the
melting point is usually between 1,200–1,600 °C.
Changing from a solid to a gas—Sublimation
Some substances can also change directly from a solid to a gas. This process is called
sublimation. One of the more popular examples of sublimation is dry ice which is frozen carbon
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dioxide (CO2). To make dry ice, carbon dioxide gas is placed under high pressure and made
very cold (about −78.5 °C). When a piece of dry ice is at room temperature and normal
pressure, the molecules of CO2 move faster and break away from each other and go directly
into the air as a gas. Regular ice cubes in the freezer will also sublimate but much more slowly
than dry ice.
Changing from a liquid to a gas—Evaporation
You see evidence of evaporation all the time. Evaporation causes wet clothes to dry and the
water in puddles to “disappear”. But the water doesn’t actually disappear. It changes state from
a liquid to a gas.
The molecules in a liquid evaporate when they have enough energy to overcome the attractions
of the molecules around them. The molecules of a liquid are moving and bumping into each
other all the time, transferring energy between one another. Some molecules will have more
energy than others. If their motion is energetic enough, these molecules can completely
overcome the attractions of the molecules around them. When this happens, the molecules go
into the air as a gas. This process is called evaporation.
Heating increases the rate of evaporation
You’ve probably noticed that higher temperatures seem to make evaporation happen faster. Wet
clothes and puddles seem to dry more quickly when they are heated by the sun or in some other
way.
You can test whether heat affects the rate of evaporation by placing a drop of water on two
paper towels. If one paper is heated and the other remains at room temperature, the water
that is heated will evaporate faster.
When a liquid is heated, the motion of its molecules increases. The number of molecules that
are moving fast enough to overcome the attractions of other molecules increases. Therefore,
when water is heated, more molecules are able to break away from the liquid and the rate of
evaporation increases.
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Different liquids have different rates of evaporation
It makes sense that different liquids have different rates of evaporation. Different liquids are
made of different molecules. These molecules have their own characteristic strength of
attraction to one another. These molecules require a different amount of energy to increase their
motion enough to overcome the attractions to change from a liquid to a gas.
Liquids will evaporate over a wide range of temperatures
Even at room temperature, or lower, liquids will evaporate. You can test this by wetting a paper
towel and hanging it up indoors at room temperature. Evaporation at room temperature might
seem strange since the molecules of a liquid need to have a certain amount of energy to
evaporate. Where do they get the energy if the liquid is not warmed? But remember that the
temperature of a substance is the average kinetic energy of its atoms or molecules. Even cold
water, for instance, has a small percentage of molecules with much more kinetic energy than the
others. With all the random bumping of a billion trillion molecules, there are always a few
molecules which gain enough energy to evaporate. The rate of evaporation will be slow but it
will happen.
Changing from a gas to a liquid—Condensation
If you have seen water form on the outside of a cold cup, you have seen an example of
condensation. Water molecules from the air contact the cold cup and transfer some of their
energy to the cup. These molecules slow down enough that their attractions can overcome their
motion and hold them together as a liquid. This process is called
condensation.
Cooling increases the rate of condensation
You can test to see if cooling water vapor increases the rate of
condensation by making two similar samples of water vapor and
cooling one more than the other. In the illustration, two samples of
water vapor are trapped inside the cups. Ice is placed on one top
cup but not the other.
In a few minutes, there are water drops on the inside top of both cups
but more water can be seen on the inside of the top cup with the ice. This shows that cooling
water vapor increases the rate of condensation. When a gas is
cooled, the motion of its molecules decreases. The number of
molecules moving slow enough for their attractions to hold them
together increases. More molecules join together to form a liquid
and the rate of condensation increases.
The amount of water vapor in the air affects the
rate of condensation
Temperature isn’t the only factor that affects the rate of condensation. At a given temperature,
the more water molecules in the air, the greater the rate of condensation. If there are more
molecules, a greater number of molecules will be moving at these different speeds. More
molecules will be moving slowly enough to condense.
Different gases condense at different temperatures
Each gas is made up of its own molecules which are attracted to each other a certain amount.
Each gas needs to be cooled to a certain temperature in order for the molecules to slow down
enough so that the attractions can hold them together as a liquid.
Changing from liquid to solid—Freezing
If a liquid is cooled enough, the molecules slow down to such an extent that their attractions
begin to overcome their motion. The attractions between the molecules cause them to arrange
themselves in more fixed and orderly positions to become a solid. This process is called
freezing.
Water molecules move further apart as water freezes
The freezing of water is very unusual because water molecules move farther apart as they
arrange themselves into the structure of ice as water freezes. The molecules of just about every
other liquid move closer together when they freeze.
Different liquids have different freezing points
It makes sense that different liquids have different freezing points. Each liquid is made up of
different molecules. The molecules of different liquids are attracted to each other by different
amounts. These molecules have to slow down to different extents before their attractions can
take hold and organize themselves into fixed positions as a solid.
Changing from a gas to a solid—
Deposition With the right concentration of gas
molecules and temperature, a gas can change directly to a
solid without going through the liquid phase. This
process is called deposition. It is the opposite of
sublimation. One of the most common examples of
deposition is the formation of frost. When there is the
right combination
of water vapor in the air and temperature, the water can change to frost without first condensing
to liquid water.
Evaporation, condensation, and the weather
Clouds- Clouds form when liquid water evaporates to become water
vapor and moves up into the sky in upward-moving air. Air at higher
altitudes is usually cooler than air near the ground. So as the water
vapor rises, it cools and condenses, forming tiny drops of water.
These droplets are suspended in the air as clouds. Clouds at higher
altitudes where the air is even colder also contain ice crystals.
Clouds at very high levels are composed mostly of ice crystals.
Rain - Rain begins as tiny droplets of water suspended
in the air as
clouds. These droplets are so small that they don’t fall yet to the
ground as rain. They are similar to the tiny droplets in fog or mist.
But when these droplets collect and join together, they become
bigger and heavier drops. Eventually these drops become so heavy
that they fall to the ground as rain.
Snow - Like rain, snow begins with condensing water vapor that
forms clouds. However, when it’s cold enough, water vapor not only
condenses but also freezes, forming tiny ice crystals. More and more
water vapor condenses and freezes on these seed crystals, forming
the beautiful ice crystals with six-sided symmetry that we know as
snowflakes.
Hail - Hail forms when a small drop of water freezes,
falls, and
then gets pushed back up into a
cloud. More water droplets collect
and freeze
on this ice crystal, which makes it heavier, and it begins
to fall again. The violent air in a thunder- cloud (cumulonimbus cloud) repeatedly bounces the
ice crystal upward. Each time it gets another coating of freezing water. Finally, the ice crystal is
so heavy that it falls all the way down to the ground as hail.
Dew -Dew is produced when moist air close to the ground cools
enough to condense and forms liquid water. Dew is different than
rain because dew doesn’t fall onto the ground in drops. It slowly
accumulates to form drops on objects near the ground. Dew often
forms on blades of grass and leaves and can make beautiful
designs on spider webs.
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Frost -If the temperature of surfaces on the ground is low enough, water vapor in the air can
change directly to the solid frost without first condensing to a liquid.
Fog -Usually the air near the ground is warmer than the air above it, but
the conditions that cause fog are just the reverse. Fog forms when warm,
moist air passes over cold ground or snow. As the water vapor in the air
cools, it condenses, forming very tiny drops of water suspended in the
air. Fog is very much like a cloud, but closer to the ground.
Mist on a Pond -Water evaporates even when the air is cold.
To form
mist, the water in a pond, pool, or hot tub must be warmer than the air
above it and the air must be cold enough to cause the water vapor to
condense as it rises. The mist seems to disappear as the water droplets
evaporate to become water
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